A. Field of the Invention
This invention refers to a method and apparatus for coating a glass substrate by ultrasonic nebulization of a solution, and more specifically to a method and apparatus for coating a hot glass substrate with a mist from a solution of a chemical precursor or mixture of precursors of one or more metal oxides in a solvent, by nebulizing the solution with ultrasonic vibration, to form the mist of fine droplets, conducting the mist to the hot glass substrate, and depositing the mist in the hot glass substrate to pyrolize it and form the desired coating.
B. Description of the Related Art
In the glass for automotive purposes, it is highly desirable that the glass have an aesthetic color, adequate visible light transmittance and low infrared (IR) and ultraviolet (UV) transmittances of the solar radiation, in order to allow the driver a good visibility of his surroundings, thus complying with the regulations of automotive safety, and controlling the accumulation of heat in the interior of the vehicles, resulting in a reduction in the consumption of energy for the automotive air conditioning equipment, and protecting vehicle materials from harmful UV radiation.
Similarly, in glass for architectonic purposes, it is highly desirable that the glass have an aesthetic color, visible light transmittance and thermal absorption properties, so as to reduce the glare and climate conditioning needs, which will result in a reduction in the consumption of electric energy for the building air conditioning equipment, also protecting plastic and fabric materials from padding DME to harmful UV radiation.
It is well known that the desirable color, visible, IR and UV transmittance characteristics of the glass can be controlled by adding several coloring agents to the glass manufacturing composition, i.e. these properties are provided directly through the glass mass.
The above disclosed properties can also be imparted to the glass by depositing coatings of organic or inorganic materials on either one or both of the surfaces of a glass sheet, which additionally provide a light, IR and UV reflecting properties which contribute to the reduction of energy consumption.
Specifically in coated glass, it is also desirable to provide the glass with an appearance and color similar to those of glass compositions containing coloring ingredients homogeneously dispersed throughout the glass.
One of the most commonly used methods to provide a suitable coating on the glass is the so known pyrolytic method. Also, There is a wide number of known techniques for preparing pyrolytic thin films by procedures and mechanisms of depositions, to provide a coating on the glass imparting the appearance, color and thermal properties similar to those of glass compositions.
Examples of methods for preparing thin films are: chemical vapor deposition; electrolytic deposition; vacuum deposition; plasma deposition; solid and liquid phase deposition, etc. Some of these methods are carried out on line during the continuous production of float glass, having their own advantages, limitations and complexity of operation.
Some pyrolytic methods involve the use of solid precursors, applied in a powder state directly on the hot glass; some other are related with the use of a solution of chemical precursors in a solvent, spraying the solution on the hot glass; and some other refer to methods for applying liquid suspensions of chemical precursors also in a sprayed form on the glass.
Examples of such patented inventions are described in the following for reference.
U.S. Pat. No. 4,397,671 issued on August 1983 to Jong and Harbor, discloses a method for applying a metal oxide film on a surface of a heated glass substrate, by spraying a heat decomposable organic-based metal salt powder on the glass ribbon. The spray powder is delivered by a gaseous stream to a location spaced above of the moving glass ribbon, and accelerated by a high velocity stream of air and directed downwardly towards the moving glass ribbon and longitudinally along the direction of movement of the glass substrate. Special emphasis is given to the size of the powder spraying, to the method for preparing the powder and to the way of how this powder is sprayed on the glass.
U.S. Pat. No. 4,393,098 issued on Jul. 12, 1983 to Stinson et al, discloses a method similar to the disclosed in the above mentioned patent, particularly referring to the use of a powder with more specific characteristics like size (about 6 microns), shape or form (spherical) of the particles and the composition of the particle itself (70% cobalt, 19% iron and 11% chromium).
U.S. Pat. No. 3,660,061 to Donley et al, discloses a method of coating a surface of a newly formed heated glass sheet with a solution of a metal oxide film forming composition, in an oxidizing atmosphere. The coating is applied by exposing the glass sheet to said composition for at least 3 seconds, immediately after the glass sheet has been removed from a reducing atmosphere containing molten tin and its vapor, while the glass surface is at a temperature sufficiently high to pyrolize the composition to form a metal oxide coating thereon of sufficient thickness to provide the coated glass with desired properties, such as transmittance of infrared and ultraviolet radiation. Care is taken to ensure that the carrier for such composition does not evaporate completely on route to the glass surface to avoid a mottle appearance.
U.S. Pat. No. 5,393,593 to Gulotta et al discloses a process for the production of a coated glass, using, as substrate, a dark gray glass having different properties that can be obtained with combination of different glass compositions and different compositions of the coating film. Such properties are mainly luminous, infrared and total solar energy transmittance which are obtained in lower values compared with values corresponding to not coated colored glass. The method for deposition of coating on the glass is by pneumatic spraying of an aqueous suspension which is prepared with certain proportions of organic based metal salt powder finally divided and suspended in water. The suspension is applied on a continuous glass ribbon at about 590.degree. C. during its production.
U.S. Pat. No. 4,719,127 to Greenberg is related also with coatings of metal oxides deposited from an aqueous suspension wherein organometallic coating reactants typically used in organic solutions are chemically suspended in an aqueous medium by the use of a chemical wetting agent in combination with extremely fine powder reactants. The organometallic coating reactants chemically suspended in an aqueous medium may be pyrolytically deposited to form metal oxide films on a hot glass substrate using conventional spray equipment, and under temperature and atmosphere conditions generally encountered in pyrolytic coating operations.
U.S. Pat. No. 2,688,565 to Raymond, and No. 4,308,319 to Michelotti et al disclose the generation of a coating composition pneumatically from an aqueous cobalt-nitrate solution. In most cases, acetylacetonate metal salts dissolved either in water but preferably in a suitable organic solvent, are used for pneumatic generation of coating compositions as it is taught by Donley in his U.S. Pat. Nos. 3,660,061 and 4,401,474, or in U.S. Pat. Nos. 4,308,319 to Michelotti and 5,356,718 to Athey et al.
Nevertheless all of such methods show important disadvantages when they are compared with the chemical vapor deposition methods in terms of versatility, adaptability, compatibility, simplicity, quality, reproducibility, productivity and cheapness.
The methods of chemical vapor deposition for the deposition of a thin film coating on one surface of a hot glass substrate, generally includes a gas or volatile liquid source; a reactant gas source; a gas mixing and distribution system; a reaction chamber and a scrubber system for exhaust gases
As metal salts, two typical groups of material are known as being usually used, i.e. the nitrate salts and more often acetylacetonates. The known art has also disclosed that spray guns based on pneumatic principle, are preferably used to supply precursors consisting of a particular coating composition and the carrier gas, such as those disclosed in U.S. Pat. No. 4,234,331 to Gray et al and in U.S. Pat. No. 5,356,718 to Athey et al.
Therefore applicants have found that, if actually there are limited amount of precursors available to be used in gaseous way, attention had to be focused to find procedures, if any, to transform pyrolyzable solutions in microdroplets, finding two principal procedures available in the market, which are used in other areas;
(1) By electrostatic and pressure nebulization, also called microsprayers, which are based on the principle of transferring an electrostatic charge to the liquid which at the exit of the atomizer nozzles is transformed in fine droplets by the tendency of repulsion of the electric charge. The higher the electrostatic charge of the liquid, the smaller the size of the micro droplets. The inconveniences of this procedure is that some liquid with higher values of conductivity, cannot retain high levels of electrostatic charges and consequently cannot be conveniently nebulized and would need the help of pressure in the liquid which increase nebulization when the liquid exits through the nozzles; and PA1 (2) By ultrasonic atomization of solutions, which is used mainly for various medical and industrial applications, as disclosed in French Patent No. 7,038,371 to Spitz et al. The patent teaches that the ultrasonic nebulization technique is particularly suitable for deposition of very thin transparent semiconductant oxides. This method, called in the Spitz's patent teaching as Pyrosol process, has many advantages if compared to the conventional pneumatic atomization techniques. PA1 It is possible to have a wide range of film compositions by providing a preparation of a liquid solution with similar viscosity than water. PA1 The very high level of surface planarity and homogeneity which can be obtained in the film by the use of very fine droplets nebulized as an aerosol by ultrasonic vibration, compared with the droplets obtained by pneumatic spraying systems or ultrasonic nozzle considered in different methods. PA1 Size diameter in droplets obtained with the ultrasonic method can be of less than 10 microns, while with pneumatic spraying systems, droplet size is normally over 100 microns with a wide range size. PA1 The nebulized aerosol can be conducted in a stream flow of gaseous air almost like another gas and then, as has been described above, in many concepts, the total deposition process can be considered as a chemical vapor deposition process with the advantages that this process offers.
This procedure has been carried out by ultrasonic nozzles which are commercially available in the market, basically consisting of a liquid delivery unit, a flow control module and the ultrasonic nozzle arrangement which includes a piezoelectric vibrator. However, in the applicant's opinion, this procedure produces a fraction of droplets bigger than the desired size and therefor has to include some means for removing them by the use of a tramp which works as a filter
The method of deposition by ultrasonic nebulization of solutions, in accordance with the present invention and which will be disclosed hereinbelow, overcomes the above disclosed limitations because it is a way to practically vaporize and apply the precursor or mixture of precursors, as a gas, to the hot glass to provide a desired coating, in a similar way as the chemical vapor deposition process.
The method of the present invention involves the application of ultrasonic vibration to a solution prepared with a chemical precursor or mixture of precursors, in a solvent, which, by a controllable ultrasonic vibration input, is controllably nebulized in a great amount of very fine liquid droplets which are conduced by a carrier gaseous stream, almost like another gas, through pipes, to a coating chamber over the hot glass substrate, in order to be deposited as a very uniform film with desired light and thermal reflective properties.
This method is particularly applicable on a continuous glass ribbon during the production, in the float glass manufacture process, at a location where the temperature is sufficiently high to form the oxide type film on the glass with convenient properties. The best location for treatment of the glass ribbon with this method is at the beginning of the annealing chamber where the temperature of the glass ribbon is 580.degree. C.-610 .degree.C.
The chemical precursors can be selected from a very wide group of available precursors provided that they can be soluble in a suitable solvent, preferably with a viscosity similar to the water and that can be pyrolyzed at elevated temperatures.
A convenient chemical precursor that can be used in the method of the present invention is the acetylacetonate of cobalt, chromium, iron, titanium, zirconium, or other chemical elements depending on the desired properties to be imparted to the glass, or mixture thereof. Organic or inorganic solvents such as methylene chloride, dimethylformamide, trichloroethylene, isopropylic alcohol, toluene, xilene, carbon tetrachloride, etc., can be used.
Condensation in the walls of the conducting pipe can be minimized by controlling the concentration of droplets of the solution in the carrier gas and also controlling the temperature of the conduction pipe.
Depending of the vapor pressure of the solvent used for preparing the solution, it is possible that during conduction of the ultrasonic nebulization, the small droplets of the solution could lose the solvent content converting the droplets to microparticles which also tend to flow in the stream of the carrier gas.
When the small droplets of the solution or the microparticles reach to the proximity of the hot glass surface, their temperature is rapidly increased vaporizing the residual solvent, so that the solid microparticles start to react with the oxygen of the carrier gas stream on the glass surface, forming the metallic oxide film. The solid particles which react prematurely in the gaseous phase tend to form reaction products which do not take part of the film and then they are removed in the flow of exhaust products together with residual air, vaporized solvent and unreacted product of the chemical precursor.
The main advantages which can be obtained with the use of the method of the present invention for obtaining a thin film of oxides on glass, are the following;
As will be described in detail below, our invention overcome the actual limitations that commonly happen when the purpose is to deposit oxide type films on glass by a traditional chemical vapor deposition technique, and besides our method is far away of methods just described in a general way above in referenced patents.